URIC ACID A NORMAL CONSTITUENT OF BLOOD Up till quite recently it was held that in normal persons the amount of uric acid in the blood was too small to be detected; also that uric acid was not demonstrable in the blood of normal individuals when on a _purin-free_ diet. On the other hand, if the subject’s blood was found to contain _uric acid_, while on a purin-free diet, it was held a characteristic feature of _gout_ and of prime diagnostic import. But, since the introduction of Folin and Denis’s method, it has been established that uric acid is constantly present, in demonstrable amounts, in _human_ blood. These authorities, using their _colorimetric_ method, found that the uric acid content of the blood ranged from 0·7-3·7 mg. per 100 grams. They believe that 1-2 mg. of uric acid per 100 grams of blood is well within the normal variations, but “are not prepared to say that they represent the full variations.” However, before applying their colorimetric method to human subjects, Folin and Denis conducted some researches into the uric acid blood content of a variety of animals, the results of which appear in the following table:— URIC ACID, TOTAL NON-PROTEIN NITROGEN AND UREA NITROGEN IN BLOOD (The Figures represent Milligrams per 100 grams of Blood.) ------------------------------------+-----+-----------+--------- |Uric |Non-protein| Urea |acid.| nitrogen. |nitrogen. ------------------------------------+-----+-----------+--------- Rabbit (6 cases) |0·05 | 31 | 13 Sheep (mixed blood) |0·05 | 28 | 13 Pig (mixed blood) |0·05 | 32 | 14 Horse (1 case, anti-toxin animal) |0·05 | 54 | 28 Monkey (1 case, poliomyelitis) |0·05 | 60 | 38 Beef (mixed blood) |0·2 | 24 | 14 Cat (2 cases, diet, liver) |0·2 | 60 | 34 Cat (2 cases, diet, milk and eggs) |0·2 | 67 | 37 Cat (2 cases, diet, rice and cream) |0·2 | 31 | 20 Chicken (6 cases, mixed blood) |4·9 | 32 | 8 Duck (4 cases, mixed blood) |4·8 | 34 | 7 Goose (1 case) |4·8 | 26 | 8 ------------------------------------+-----+-----------+--------- The most striking feature of these findings is the marked contrast between the uric acid blood content of _mammals_ as opposed to avians. In the former the amount is minimal—0·2 mg. or less per 100 grams of blood. On the other hand, in the blood of _birds_, in whose instance the origin of uric acid is so different, it is present in relatively large amounts. As Folin and Denis observe, the small amounts of _urea_ in the blood of birds, as compared with that of mammals, is also worthy of note.